Way back on April 16, 1862 President Abraham Lincoln signed a bill that ended slavery in the District of Columbia. He actually signed this bill 9 months before he signed the Emancipation Proclamation.
So what does this have to do with taxes you ask? According to Federal law, holidays observed in DC affect the entire nation. So. April 15, the normal Day of Darkness (tax day), falls on a Sunday so you get a reprieve there, and Monday, April 16, is Emancipation Day in DC -- therefore, tax day is April 17 this year.
Of course the brilliant folks in DC didn't figure this out until they already printed a bunch of forms showing a deadline of April 16. So, as long as you are the type of person whose tax deadline normally falls on April 15, you don't have to mail in your return until April 17.